Or would it be easier to find a servicable car along the road, take it and then be able to park right next to a car they can siphon from? Sure, there could be walkers in among the trees, but they are easy to pick off if you have a look out, and except when a herd is rolling through, aren't usually in large, concentrated numbers like was seen in the town scenes.
BTW, larger car dealerships here in GA have their own buried underground gas tanks.
That, and gas stations would be tough since there is no electricty, and the gas is to low for syphoning. You would have to to either have generator supply power to the pump, or better yet have a gas powered pump to get the gas out. Heck, you could even put it in a tanker truck than you would be good for a while.
If Walter White can rob 1000 gallons of methlemine in minutes unnoticed, I'm sure rick and the gang can get some gas! LOL